The present invention relates to a vacuum pumping system suitable for pumping low thermal conductivity gases, such as argon and xenon.
Extreme Ultra Violet Lithography (EUVL) extends the current technology of optical lithography by using wavelengths in the range 11 to 14 nm, in order to shrink the size of printable features in the manufacture of integrated circuits. At these wavelengths all materials are strongly absorbing, and therefore this type of lithography must be performed under vacuum.
The source for EUV radiation may be based on excitation of tin, lithium, or xenon. The use of metallic materials such as tin and lithium presents the difficulty that these materials may be evaporated and become deposited on sensitive optical components. Where xenon is used, light is generated in a xenon plasma either by stimulating it by an electric discharge or by intense laser illumination. Because the EUV radiation has very poor transmissibility through xenon, it is necessary to reduce the pressure in the area around the plasma using a vacuum pumping system. However, pumping the quantities of xenon required (up to 10 slpm at 1×10−2 mbar) for the production of the plasma with conventional turbo-molecular pumps is not possible.
From first principles, work is done when a gas is compressed, or expanded. The process can be considered adiabatic in a well-insulated system or where the process is so rapid that there is not enough time for appreciable heat transfer to take place. As a gas is compressed, its temperature increases as work is being done to it, increasing its internal energy. For expansion, the adiabatic process is reversed and the temperature decreases.
For an ideal gas the specific heat capacity at constant pressure is given by Cp=Cv+R, where Cv is the molar specific heat capacity at constant volume, and R the specific gas constant. The ratio of specific heats (or the molar heat capacity) of a monatomic gas is given by.γ=Cp/Cv=(5R/2)/(3R/2)=5/3.
A mechanical vacuum pump and the gas being pumped can be considered as a closed thermodynamic system. The pump takes a body of gas and compresses it, allows it to expand, and exhausts it to atmosphere. In the simplistic case of assuming adiabatic compression, the volumetric ratio of inlet to outlet is given by
                                          V            1                                V            2                          =                              (                                          p                2                                            p                1                                      )                                1            /            γ                                              (        1        )            
The outlet temperature T2 is given by
                              T          2                =                                                            T                1                            ⁡                              (                                                      V                    1                                                        V                    2                                                  )                                                    γ              -              1                                ⁢                                          ⁢          or                                    (        2        )                                          T          2                =                                            T              1                        ⁡                          (                                                p                  2                                                  p                  1                                            )                                            γ            -                          1              /              γ`                                                          (        3        )            
Xenon is monatomic and has a high molar heat capacity (γ=1.667) combined with low thermal conductivity (making it a good insulator). The molar heat capacity and the thermal conductivity of a gas are related to its molecular structure. The atomic mass (131.29 amu) and radius (108 pm) of xenon is greater than that of argon (39.95 amu and 98 pm, respectively). Some properties of xenon, argon, helium and nitrogen are given in Table 1 below for comparison.
TABLE 1XeArHeN2Atomic number541827Atomic mass,131.2939.9484.00314.01amuAtomic radius,131884975pmGas density,5.54,1.784,0.1785,1.251,(liquid density),(3057)(1394)(122)(806.5)kg/m3Ratio of molar1.6671.6671.6671.4heat capacities,(γ)Tcrit, ° C. @ atm16.6, (8° C.(−122° C.−267.96(−146.9° C.@ 50 bar)@ 50 bar)@ 50 bar)Tboil, ° C.−108−186−268.785−195.8Tmelt, ° C.−111.7−189.3−272.05−210.1Thermal0.005650.017720.140.02583conductivity,W/mK
From equation (3) above, even for a moderate vacuum (0.1 mbar), the outlet temperature of the gas would be considerable. Ordinarily, for diatomic gases or those with higher thermal conductivities and smaller atomic masses, the fact that the gas expands before being exhausted from the pump would result in a considerable temperature reduction. However, xenon is averse to relinquishing its newly acquired heat energy.
The difficulty in pumping xenon with a turbo-molecular pump occurs primarily at the inlet of the pump. The first stage comprises an axial compressor made up of rotating blades separated by stationary blades. They operate under molecular flow conditions and the incident of the rotor blades is designed to encourage the molecules axially through the stages down to the exhaust or high-pressure end of the pump. The rapidly rotating blades of the turbo-molecular pump hit the molecules of gas in the chamber. This collision transfers some momentum to the particles. This process of momentum transfer is more efficient if the average linear velocity of the molecule is less than the linear velocity of the blade tip. For a xenon molecule, the average velocity at 27° C. is 318 m/s. However, the larger the mean blade diameter of the pump, the higher the tip speed. Generally small turbo-molecular pumps (<500l/s N2) are designed to run at very high speeds (>50,000 rpm) and the larger pumps (>1000l/s N2) run at slower speeds (<30,000 rpm) in order to pump the light gases, as the efficiencies of turbo-molecular pumps are greatest for the heavier gases. Xenon molecules are “heavy” by comparison to lighter gases and therefore move more slowly through the pump. As work is being done on the heavy xenon molecules, their internal energy is increased and heat is produced. As the metal impeller has a high thermal conductivity, this heat is conducted through the impeller rapidly whilst the static component remains cold. For effective molecular pumping, the clearances between the rotor and stator must be of the order of microns. In some cases, the thermal expansion of the rotor, differentially from the stator, causes failure.
Some pumps are also designed with a “self-cooling” back leakage from the exhaust over the stator and rotor seating. This works to the detriment of the pump in the case of xenon, as the already hot gas now re-circulates in the back of the pump, which gets progressively hotter. This is further aggravated by the insulating nature of the gas, which holds onto the heat energy.
Typically, improvement of the pumping process is carried out by the use of a purge gas lighter than xenon in the turbo-molecular pump. On average lighter gas molecules, like N2 and He, travel faster than heavier gases (e.g. Xe). Therefore, these gases have a higher impingement rate on the walls of a chamber or on the blades of the turbo-molecular pump, but they also have smaller momentum. The average speed ({overscore (v)}) of a gas molecule is dependant upon the mass (M) of the molecule and temperature (T), as set out below.
                              v          _                =                                                            8                ⁢                                                                  ⁢                                  R                  0                                ⁢                T                                            π                ⁢                                                                  ⁢                M                                              ⁢                      (                          m              /              s                        )                                              (        4        )            
For example, at room temperature the average speed of molecules of He, N2, and Xe, are 1245 m/s, 470 m/s, and 215 m/s, respectively. The higher the temperature, the greater the average speed, and the average speed will be greatest for the gas whose molecules have the least mass. As He (k=0.14 W/mK) has a considerably greater thermal conductivity than Xe (0.00565 W/mK), the He molecules would aid the transfer of heat from the pump and the Xe gas. This can maintain the temperatures inside the pump at levels that allow reliable pump operation for much longer periods than would be possible in the absence of a light purge gas.
As xenon occurs in atmospheric air in very low concentrations (around 0.087 ppm), the cost is very high. It is therefore very desirable to recover and re-use the xenon. One method available for the recovery of xenon is the use of a low temperature (cryogenic) trap to freeze the xenon while permitting the noncondensable light purge gas to pass through the trap and be vented to atmosphere. Once the trap has captured a sufficient amount of xenon, it can be regenerated by heating, which vaporizes the xenon so that it can be collected separately.
However, the presence of the purge gas in the pumped xenon stream makes the purification and subsequent recycle of the xenon particularly complex and costly. For example, suppose that the flow rate of the xenon being pumped out of the chamber is 0.4 slpm. Suppose also that a light purge gas, say N2, is added to the turbo-molecular pump at a flow rate of 3.6 slpm. The pump output is now 4.0 slpm at 10−3 bar with 90% N2 and 10% Xe therein (pXe=10−4 bar). If the cryogenic trap to which this gas mixture is fed is operated using liquid nitrogen at or slightly above ambient pressure as the refrigerant, the operating temperature of the trap could be as low as −192° C. The vapour pressure of xenon at this temperature is about 10−5 bar. Thus, the noncondensable N2 gas leaving the trap at 10−3 bar takes with it xenon at 10−5 bar (Xe content is thus 1%). This outlet stream thus has a flow rate of 3.6364 slpm, with 99% N2 (molar flow is still 3.6 sipm) and 1% Xe. Note that the molar flow rate of xenon in this stream is about 0.0364 slpm, which represents more than 9% of the xenon extracted from the vacuum chamber (0.4 slpm). This loss of xenon would be much higher if the trap could not be operated at such a low temperature. If this 9% or higher loss of xenon on a continuous basis is acceptable, a simple cryogenic trap operated in a conventional scheme is sufficient for xenon recycle, with the light purge gas with the uncaptured xenon in it being rejected as waste from the system. However, in the application of xenon to EUV lithography, the economics do not allow for such a high wastage of xenon.
It is an aim of the present invention to provide a more cost-effective apparatus for, or a method of, pumping low thermal conductivity gases, such as argon and xenon.